base.bbclass: fix do_unpack[cleandirs] varflag handling

As introduced by a56fb90dc3 ('base.bbclass
wipe ${S} before unpacking source') the base.bbclass uses a python
anonymous function to set the 'do_unpack' varflag 'cleandirs' to either
'${S}' or '${S}/patches' depending on equality of '${S}' and '${WORKDIR}'.

Not that this only differs from the way almost all other recipes set or
modify a tasks 'cleandirs' flag, it also has a significant impact on the
kernel.bbclass (and possibly further ones) and causes incorrect
behavior for rebuilds triggered by source modification, e.g. by a change
of the defconfig file for a kernel build.

The kernel.bbclass tries to extend do_unpack[cleandirs]:

| do_unpack[cleandirs] += " ${S} ${STAGING_KERNEL_DIR} ${B} ${STAGING_KERNEL_BUILDDIR}"

As python anonymous functions are evaluated at the very end of recipe
parsing, the d.setVarFlag('do_unpack', 'cleandirs', '${S}') statement in
base.bbclass will overwrite every modification to cleandirs that is done
as shown for the kernel class above.

As a result of this, a change to a kernels 'defconfig' will lead to an
updated defconfig file in ${WORKDIR}, but as ${B} never gets cleaned and
${B}/.config still exists, it will not be copied to ${B}/.config and
thus not find its way in the build kernel.

This is a severe issue for the kernel development and build process!

This patch changes setting of the cleandirs varflag in base.bbclass to
a simple variable assignment as almost all other recipes do it. This now
again allows overwriting or appending the varflag with common methods
such as done in kernel.bbclass.

This issue affects morty, pyro, rocko and master.

(From OE-Core rev: 20901b9783220aa6e7adae4951c531919c20859b)

Signed-off-by: Enrico Jorns <ejo@pengutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/base.bbclass

@@ -152,12 +152,8 @@ python base_do_fetch() {
 addtask unpack after do_fetch
 do_unpack[dirs] = "${WORKDIR}"
 
-python () {
-    if d.getVar('S') != d.getVar('WORKDIR'):
-        d.setVarFlag('do_unpack', 'cleandirs', '${S}')
-    else:
-        d.setVarFlag('do_unpack', 'cleandirs', os.path.join('${S}', 'patches'))
-}
+do_unpack[cleandirs] = "${@d.getVar('S') if d.getVar('S') != d.getVar('WORKDIR') else os.path.join('${S}', 'patches')}"
+
 python base_do_unpack() {
     src_uri = (d.getVar('SRC_URI') or "").split()
     if len(src_uri) == 0: